A hard disk drive (hereinafter referred to as HDD) in recent years rotates a magnetic disk and drives a head stack assembly (hereinafter referred to as HSA) at high speed in response to the request for high capacity and high recording density, additionally high-speed accessing. As schematically illustrated in a side view of FIG. 8(a), the HSA includes a carriage 160 and head gimbal assemblies (hereinafter referred to as HGAs) 151 and pivots around a pivotal axis.
The HGA 151 is an assembly of a suspension and a head slider. Typically, the HSA has a plurality of arms 111. In FIG. 8, a top end arm 111a and one of the inner arms 111b are exemplarily designated with reference numerals. An inner arm is provided between two magnetic disks and two HGAs corresponding to the respective magnetic disks are attached to it. Each of the HGAs are attached to the top and bottom end arms.
With finer track pitch for high-capacity and high recording density and speed-up of driving, required conditions for vibration of the HSA has become severer. To this end, several ways to suppress vibration of the HSA at driving have been suggested. In one of those ways, a dumping member is attached to the carriage of the HSA to suppress sympathetic vibration of the carriage (for example, refer to Japanese Patent Publication No. 2003-022631 “Patent Document 1”). Covering a part of the arm with the dumping member suppresses the vibration of the arm and improves accuracy of head positioning.
Thus, to damp the vibration of the HSA, especially the vibration of the arm, it is effective that the dumping member is adhered to the arm. FIG. 8(b) is a side view schematically showing a partial configuration of the inner arm 111b to which the damping member 108 is attached. As shown in FIG. 8(b), the damping member 108 is preferably to be adhered to a single surface of the arm 111b in order to limit increase in cost and mass of the arm 111b. The damping member is attached to a single surface of each arm shown in FIG. 8(a).
The upper HGA is attached to an HGA attaching surface 213 and the lower HGA is attached to an attaching surface 214. The damping member 108 is attached to a damping member attaching surface 211. The shape of the inner arm 111b is symmetric about the center between the magnetic disks (HGAs) as illustrated in FIG. 8(b). The measurements (thicknesses) from the center line to the upper and lower HGA attaching surfaces 213 and 214 are the same and denoted by T3. Similarly, the measurement (thickness) from the center line to the damping member attaching surface 211 and the measurement (thickness) from the opposite surface 212 of the damping member attaching surface 211 to the center line are the same and denoted by T1. The thickness of the damping member 108 is denoted by T4.
In order to confirm the damping effect in the arm before and after the damping member is adhered, frequency responses are obtained by a numerical analysis simulation. FIG. 9(a) shows amplitude of the head slider in the off-track direction in the case that a periodic exciting force in the in-plane direction is applied to a tip end of the inner arm without the damping member attached. FIG. 9(b) shows the amplitude of the head slider in the off-track direction in the case that the same input is applied to the tip end of the inner arm with the damping member attached.
FIG. 10(a) shows the amplitude of the head slider in the off-track direction in the case that a torsion moment about the longitudinal direction of the arm as the axis is applied to the tip end of the inner arm without the damping member adhered. FIG. 10(b) shows the amplitude of the head slider in the off-track direction in the case that the same input is applied to the tip end of the inner arm with the damping member adhered.
During vibration of the arm (head slider), there is a sway mode in which the arm vibrates in the in-plane direction of the magnetic disk and a torsion mode in which the arm vibrates like twisting, other than a bending mode in which the arm vibrates in the normal direction of the magnetic disk. The solid line represents the vibration of the upper head slider and the dotted line represents the vibration of the lower head slider. With regard to the outputs, the peak value of the output before the damping member is adhered has been set to coincide with the actual measured value.
As shown in FIGS. 9(a) and 9(b), the vibration in the sway mode is much reduced with respect to the arm to which the damping member is adhered. On the other hand, as shown in FIGS. 10(a) and 10(b), when the torsion moment is applied, the vibration changes significantly by adhering the damping member. Specifically, in the sway mode, the vibration is excited only by an exciting force in the in-plane direction before the damping member is adhered as shown in FIGS. 9(a) and 10(a). However, after the damping member is adhered, a peak in the sway mode also appears in response to the torsion moment.
Also, as shown in FIG. 9(a), the peak values in the torsion mode of the upper and the lower head sliders coincide with each other before the damping member is adhered. However, after the damping member is adhered, a big difference appears in the peak values in the torsion mode between the upper and the lower head sliders.
These changes of the vibration are considered to be caused by the change of the mode configuration by the damping member adhered on a single surface of the arm. Adhering the damping member results in addition of mass and stiffness. This results in the change of the mode configuration of the arm. For example, the sway mode is accompanied by torsion in the mode configuration, and as shown in FIG. 10(b), an in-plane vibration (sway mode) is excited in response to the torsion excited force. In the torsion mode, there arises a difference between the amplitude of the upper and the lower head sliders so that the amplitude of one of the head sliders may not be reduced. Such vibration change disturbs accurate head positioning to result in preventing proper reading/writing processes.
In another aspect, adhering the damping member to a single surface of the inner arm results in that the clearances between the top or the bottom surface of the arm and the magnetic disk thereover or thereunder respectively are different. That is, the clearance between the surface of the damping member and the surface of the magnetic disk is smaller than the clearance of the opposite surface by the thickness of the damping member. Hence, the shock resistant performance of the HDD is considered to be degraded by the thickness of the damping member.